1. Field of the Invention
The present invention relates to a communication system for performing communication by synchronizing framed incoming data.
2. Description of the Related Art
An SDH (Synchronous Digital Hierarchy) technology being a net synchronization system to supply a synchronous clock in a network required for multiplexing has been standardized by ITU-T (International Telecommunication Union-Telecommunication), based on a SONET (Synchronous Optical Network) technology developed in the United States as high-speed transmission technology using optical fibers. An SDH signal to be sent or received in a transmission system using the SDH technology is to be frame-structured. In the frame-structured SDH signal, a fixed frame synchronization pattern is arranged at a specific position of a frame on a sending end, and the frame synchronization pattern is recognized on a receiving end to perform a synchronous detection of a frame, so that a frame phase is exactly recognized, and a signal in a predetermined format is sent and received.
Here, a receiving end device receives an optical signal and processes it as an electrical signal. However, in a communication system in which a high-speed optical signal, such as at 40 Gb/s, is transmitted via an optical fiber, it is difficult for the receiving end device to process the optical signal electrically at this speed. Therefore, in such a communication system, a high-speed signal is converted into a lower speed parallel signal to be processed electrically (refer to, for example, Japanese Patent Application Laid-open No. 2004-112123).
FIG. 8 shows one example of such communication systems. A transmitting device 101 and a receiving device 102 composing a communication system 100 are connected with an optical fiber 103. An optical signal 104 is sent at 40 Gb/s from the transmitting device 101 to the receiving device 102 through the optical fiber 103.
At the receiving device 102, a receiving side optical module 111 converts the optical signal 104 into an electric signal, and sends it to a signal processing unit 114 as a parallel data 113 composed of a zeroth to fifteenth signals 1120-11215. At that time, a deskew signal 115 is generated in the receiving side optical module 111 for synchronizing each of the zeroth to fifteenth signals 1120-11215 in frames. The deskew signal 115 is a signal for correcting a phase shift of the parallel data 113, and extracted partially from each signal received by the receiving side optical module 111. The deskew signal 115 is sent to the signal processing unit 114.
At the signal processing unit 114, the zeroth to fifteenth signals 1120-11215 received are compared in frames with a frame synchronization pattern which is determined according to the deskew signal 115. Further, even if the patterns coincide, the synchronization is not established immediately. The synchronization is not established until a coincidence of the patterns is detected continuously in a plurality of frames. In this case, the synchronization is established when the coincidences are detected in continuous “a” number of frames (“a” is 2 or a larger positive integer). It is expressed as “a synchronization establishment protection stage count is “a” stages”. For example, when the synchronization establishment protection stage count is assumed to be 2 stages, it means that, in the fifteenth signal 11215, as an example, the frame synchronization patterns coincide in sequential two frames.
In the communication system 100 shown in FIG. 8, an early stage immediately after the transmitting device 101 is powered or the receiving side optical module 111 in the receiving device 102 is powered, is considered. The system is not stable in such a stage. In such an unstable stage, as a bit length composing the frame synchronization pattern, a bit string having at least an all-zero or all-one is more likely to be generated, as zeroth to fifteenth signals 1120-11215. The same problem also occurs when there is a failure such that the optical signal 104 is not sent to the receiving device 102 temporarily because of the disconnection of the optical fiber 103.
If the signal processing unit 114 monitors whether there is a coincidence of the patterns or not by varying the frame synchronization pattern in frames according to the deskew signal 115, the frame synchronization pattern is more likely to be the aforementioned all-zero state in a condition where the system is not yet started up normally. Further, in such a condition, the all-zero bits is more likely to continue in the frames of the zeroth to fifteenth signals 1120-11215. Accordingly, a synchronization of frames is more likely to be detected wrongly in such a case.
Once the frame synchronization is detected wrongly, as in a same manner with a state shifting from non-synchronization state to synchronization establishment state as described above, the protection stage count is generally set also in a state shifting from the synchronization establishment state to the non-synchronization state. A non-synchronization protection stage count is assumed to be “b” stages (“b” is 2 or a larger positive integer). For example, when the non-synchronization protection stage count is assumed to be three stages, it means that the frame synchronization pattern does not coincide in sequential three frames of the fifteenth signal 11215.
Once an unillustrated frame synchronization device in the signal processing unit 114 in FIG. 8 is in an erroneous synchronization state, (b+a) number of frames, which is summation of “b” stages of the non-synchronization protection stage count and “a” stages of the synchronization establishment stage count thereafter, are required to be received by the receiving device 102 from receipt of a normal frame synchronization pattern to completion of normal synchronization pull-in. That is, the frame synchronization is wrongly detected in some cases when a system is in an abnormal state, and in such a case, there is a problem in which it takes comparably long time for the system to be recovered from a line disconnection status to a normal status. This is a serious failure for a device, such as a SONET/SDH device, in which particularly high-quality is required for a line.
Performance degradation in the SONET/SDH device has been explained hereinbefore. Generally, there is the same problem also in a frame synchronization device and a frame synchronization method which synchronizes a frame by using a frame synchronization pattern extracted partially from a signal sent by a transmitting end.